1. Field of the Invention
The invention relates to a receiver for frequency modulated signals having two quadrature channels. Each channel comprises a synchronous demodulator followed by a low-pass filter and a differentiator. The output signal of each of the low-pass filters is multiplied by the differentiated output signal of the filter of the other channel. The output signals of the multipliers are subtracted from one another. The output signal thus produced is proportional to the frequency difference between the received signals and the reference signal applied to the synchronous demodulators.
In such a receiver filtering of the signals is effected by the low-pass filteres and does therefore not depend on the nominal centre frequency of the received signals. The operating frequency of the receiver is determined by the frequency of the reference signals.
Such receivers can be used for, for example, data transmission over telephone lines by means of FSK.
2. Description of the Prior Art
A receiver of the type defined above is known from U.S. Pat. No. 3,568,067.
The omission of a tuned input filter also has its drawback. In the first plate it renders the use of a limiter in the input stage of the receiver impossible. Harmonics of channels of a lower frequency, which come close to the reference frequency would then also be received, which is unwanted. Furthermore strong neighbouring channels may push away a desired weaker channel (the limiter favours the stronger channel). In the second place harmonics of the reference signal enable also the reception at other frequencies. This drawback can be reduced by using a very pure (having a low harmonic content) sinusoidal oscillation for the reference signal.
The absence of a limiter is experienced in particular in links having a highly variable or unknown signal level. The reliability of the signal reception wil then be negatively affected.
Netherlands Patent Application No. 7800249, which has been laid open to public inspection, describes a receiver of the present type in which the two quadrature channels include control amplifiers whose gain factors are controlled via an error-signal amplifier to which the sum of the squares of the output signals of the control amplifiers is applied. This results in signals of controlled amplitude after the control amplifiers. By means of such an automatic gain control, AGC, the dynamic range is reduced, while amplitude variations persist and for reasons of stability the control speed is limited.